There is, mobile agents typically have the word mobile in the user agent string.

You can also test for if the client supports touch, but with Windows 8.1, a lot of browsers now support touch, even if they are not on mobile.

Best case would be to run some JS very early in the page and load the correct CSS for the device.

Alternately, you can set up the network to redirect mobile enabled clients to a mobile specific site via Apache rewrite rules.

http://www.howtoforge.com/apache2-how-to-redirect-users-to-mobile-or-normal-web-site-based-on-device-using-mod_rewrite